The present disclosure relates to a battery pack that can be applied to an electronic instrument, an electric vehicle, an electric tool or the like. Furthermore, the present disclosure relates to an electricity accumulation system to which the battery pack is applied, an electronic instrument, an electric vehicle, and an electric power system.
A lithium ion secondary battery using doping and de-doping of lithium ions has been used. A plurality of battery cells (also referred to as single batteries or cells) of the lithium ion secondary battery are connected to form a battery cell group. The battery cells constituting the battery cell group are connected to each other to form a battery module. One or more battery modules are housed in an exterior case to form a battery pack. The battery pack of the lithium ion secondary battery has been widely used in electronic instruments such as personal computers and mobile phones, electric vehicles, electric tools or the like.
When moisture enters the battery pack, it causes breakdown of a battery cell. Accordingly, it is desirable to improve waterproofing properties of the battery pack. As mentioned below, Japanese Patent Laid-open Publication No. 2006-164655 discloses a battery pack which seals a case receiving the battery cell to prevent moisture from entering from the outside.